The Arms Altar is a feature in Kid Icarus: Uprising that allows you to purchase weapons by redeeming in your hearts, grind up weapons for hearts, and fuse weapons together to create stronger weapons.
 Redeeming Hearts
If you choose this option, then there will be a list of weapons that have different stats, values, and attributes. Each weapon costs hearts, depending on the value of it.
 Converting to Hearts
If you choose this option, a list of all your weapons will come up, and beside them will be an amount. If you choose to grind up the weapons, you will gain that amount of hearts.
 Fusing Weapons
If you choose this option, you will be able to fuse two weapons together to create another weapon, in a process known as Weapon Fusion.